Expt:The change in mass when magnesium burns

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Introduction

Magnesium is weighed and then heated in a crucible. It reacts with oxygen to produce the oxide. It can be shown that there has been an increase in mass. The results can be used to find the formula of magnesium oxide and two methods are described for doing so.

Apparatus and chemicals

Per pair or group of students:

• Eye protection
• Crucible with lid
• Tongs
• Pipe clay triangle
• Bunsen burner
• Tripod
• Heat resistant mat
• Emery paper (optional)
• Magnesium ribbon (Low hazard), about 10-15 cm (see note 1)
• Access to:

• Balance (2 d.p.)

Technical notes

Magnesium ribbon (Low hazard) Refer to CLEAPSS® Hazcard 59A

1. Fresh, clean magnesium is best for this experiment. If the magnesium is tarnished then emery or sand paper will be required to clean it.

Procedure

Health & Safety

Wear eye protection.

Procedure

A. Cut a piece of magnesium about 10-15 cm long. If it is looking tarnished or black then clean it using the emery paper. Twist it into a loose coil.

B. Weigh the crucible with the lid (mass 1) and then the magnesium inside the crucible with the lid (mass 2).

C. Set up the Bunsen burner on the heat resistant mat with the tripod. Place the pipe clay triangle over the tripod in a ‘star of David’ formation, ensuring that it is secure. Place the crucible containing the magnesium in the pipe clay triangle and put the lid on.

D. Light the Bunsen burner and begin to heat the crucible. It is best to start with a gentle blue flame, but you will need to use a roaring flame (with the air hole fully open) to get the reaction to go.

E. Once the crucible is hot, gently lift the lid with the tongs a little to allow some oxygen to get in. You may see the magnesium begin to flare up. If the lid is off for too long then the magnesium oxide product will begin to escape. Don't let this happen.

F. Keep heating and lifting the lid until you see no further reaction. At this point, remove the lid and heat for another couple of minutes. Replace the lid if it appears that you are losing some product.

G. Turn off the Bunsen burner and allow the apparatus to cool.

H. Re-weigh the crucible with lid containing the product (mass 3).

I. Heat the crucible again for a couple of minutes and once again allow to cool. Repeat this step until the mass readings are consistent. This is called ‘heating to constant mass.’

Reference

This experiment has been reproduced from Practical Chemistry: http://practicalchemistry.org/experiments/the-change-in-mass-when-magnesium-burns,207,EX.html

Useful resource

See also: The combustion of iron wool experiment